1. Field of the Invention
This invention relates to acoustic logging within a wellbore and more particularly to determination of acoustic velocity in a drill string.
2. Description of the Prior Art
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. Presently a technique commonly known as Vertical Seismic Profiling (VSP) is utilized to image the subsurface in the vicinity of the borehole. With VSP, typically, a source is activated at the surface and sensors are positioned at discrete borehole depths with a wireline. More recently methods have been developed for performing VSP with a downhole source and with receivers positioned on the surface. Data can be acquired simultaneously at many surface locations with little more expense than for a single location. One such method utilizes the drill bit as the downhole source. See, for example, U.S. Pat. No. 4,718,048 and U.S. patent application No. 939,359, having a filing date of Dec. 8, 1986. In this method, a sensor is positioned near the top of the drill string to record a reference or pilot signal and field sensors are positioned at selected locations in the vicinity of the wellbore. The signal generated by the drill bit travels up the drill string to the reference sensor and also propagates from the drill bit upward to the field sensors and also downwardly to reflecting subsurface interfaces and back to the field sensors. By correlating the signal detected by the pilot sensor with the signal detected by the field sensors, the travel times of the energy traveling from the drill bit to the field sensors may be determined. In order for this travel time to be determined, however, the acoustic velocity of the drill string must be known. In general, the acoustic velocity of the metals comprising the drill string is known, so that the acoustic velocity may be calculated fairly accurately without actually measuring the velocity in the wellbore. However, the actual acoustic velocity of drill pipe may vary slightly because of differences in temperature and pressure downhole and because of differences in dimensions and constituent metals of the drill pipe. Accordingly, it is desirable to measure the actual velocity of the acoustic signal in the drill string.